What are the differences between flat heads, countersunk heads, and pan heads of screws?

Screws are the general term for fasteners, referring to threaded fasteners with various structural shapes.   Screws include screws, bolts, studs, and other similar fasteners.

Screws can be classified by head type into: disc head, round head, cylindrical head, countersunk head, flat round head, semi-countersunk head, semi-round head, hexagonal head, spherical cylindrical head...

Flat-head screws are less commonly used in daily life and are generally more often employed as screws in construction sites and other projects.   The head of a flat-head screw is flat, and the groove shapes include cross grooves, hexagon sockets, etc.

The contact surface between the flat head screw and the workpiece is a horizontal plane. The force-bearing surface of the flat head screw is relatively large. During installation, the flat head screw can be exposed or submerged in the hole.

Cross countersunk screws, by the national standard GB819-85, are often referred to as flat head screws by many people.  Countersunk and flat heads are not the same.  A countersunk head only has a flat surface and an inverted conical head.  When connecting, a 90-degree conical hole needs to be machined first.  After installation, it should be completely flush with the surface of the workpiece.